1. Field of the Invention
The present invention relates to a chair whose backrest can be tilted rearward.
2. Description of the Related Art
Conventionally, there has been disclosed a chair in which a backrest surface is made up by extending an upholstery member between frame elements, which make a pair on both right and left sides. For example, there has been disclosed a chair employing a structure in which an upper portion of the backrest surface is supported by upper frame elements and a lower portion of the backrest surface is supported by lower frame elements, and these upper frame elements and lower frame elements are made independent to perform rotational operation around a horizontal axis, that is, rocking movement (for example, refer to Japanese Patent Application Laid-Open No. 2002-119375 and Japanese Patent Application Laid-Open No. 2002-119373). Furthermore, in the chair having such a constitution, by providing elastically biasing means that elastically biases the lower frame elements forward and elastically biasing means that biases the upper frame elements forward, independently of each other, the backrest surface capable of following, for example, a motion of the entire upper body of a sitter by the motion of his or her waist, and for example, a motion of only an upper portion of the upper body such as turning back and extending his or her arm laterally is realized.
In everyday life, when seated with his or her upper body standing in a chair, a sitter often turns back, extends his or her arm, twists himself or herself and so on. With respect to such motions of the sitter, in the chairs described in the Japanese Patent Application Laid-Open No. 2002-119375 and Japanese Patent Application Laid-Open No. 2002-119373, the upper frame elements follow the motions of the upper body of the sitter, particularly the upper portion with a required elastically biasing force.
However, when the sitter tilts his or her upper body rearward, the backrest is required to surely support the upper body of the sitter. That is, the elastically biasing force of such a degree that the upper frame elements of the backrest can move in accordance with the motion of the upper body of the sitter as described above does not allow the upper portion of the upper body to be stably supported. In such a chair, there may arise a defect that when the sitter inclines his or her upper body by tilting the lower frame elements of the backrest rearward, the sitter feels insecure.